Bugzilla

According to [the explainer](https://wicg.github.io/aom/aria-reflection-explainer.html#multi-valued-array-relationship-attributes-are-supported), we should be able to set attributes programmatically for `element.ariaLabelledByElements` and then get back the references using `getAttribute()` - provided the referenced elements are in the DOM and have an id.

This does not appear to work, at least on MDN. This code does not log the references:
```html
<span id="label_1">Street name</span>
<input />
<span id="label_2">(just the name, no "Street" or "Road" or "Place")</span>
```

```js
const inputElement = document.querySelector("input");
const label1Element = document.querySelector("#label_1");
const label2Element = document.querySelector("#label_2");

// Feature test for ariaLabelledByElements
if ("ariaLabelledByElements" in Element.prototype) {
  inputElement.ariaLabelledByElements = [label1Element, label2Element];

  log(`Labeled elements: ${inputElement.getAttribute("aria-labelledby")}`);
}
```
You can see this in action as the second example in https://pr38678.review.mdn.allizom.net/en-US/docs/Web/API/Element/ariaLabelledByElements#assign_the_accessible_name_programmatically

According to [the explainer](https://wicg.github.io/aom/aria-reflection-explainer.html#multi-valued-array-relationship-attributes-are-supported), we should be able to set attributes programmatically for `element.ariaLabelledByElements` and then get back the references using `getAttribute()` - provided the referenced elements are in the DOM and have an id - "sprouting".

This does not appear to work, at least on MDN. This code does not log the references:
```html
<span id="label_1">Street name</span>
<input />
<span id="label_2">(just the name, no "Street" or "Road" or "Place")</span>
```

```js
const inputElement = document.querySelector("input");
const label1Element = document.querySelector("#label_1");
const label2Element = document.querySelector("#label_2");

// Feature test for ariaLabelledByElements
if ("ariaLabelledByElements" in Element.prototype) {
  inputElement.ariaLabelledByElements = [label1Element, label2Element];

  log(`Labeled elements: ${inputElement.getAttribute("aria-labelledby")}`);
}
```
You can see this in action as the second example in https://pr38678.review.mdn.allizom.net/en-US/docs/Web/API/Element/ariaLabelledByElements#assign_the_accessible_name_programmatically

According to [the explainer](https://wicg.github.io/aom/aria-reflection-explainer.html#multi-valued-array-relationship-attributes-are-supported), we should be able to set attributes programmatically for `element.ariaLabelledByElements` and then get back the references using `getAttribute()` - provided the referenced elements are in the DOM and have an id - "sprouting".

This does not appear to work, at least on MDN. This code does not log the references:
```html
<span id="label_1">Street name</span>
<input />
<span id="label_2">(just the name, no "Street" or "Road" or "Place")</span>
```

```js
const inputElement = document.querySelector("input");
const label1Element = document.querySelector("#label_1");
const label2Element = document.querySelector("#label_2");

// Feature test for ariaLabelledByElements
if ("ariaLabelledByElements" in Element.prototype) {
  inputElement.ariaLabelledByElements = [label1Element, label2Element];

  log(`Labeled elements: ${inputElement.getAttribute("aria-labelledby")}`);
}
```
You can see this in action as the second example in https://pr38678.review.mdn.allizom.net/en-US/docs/Web/API/Element/ariaLabelledByElements#assign_the_accessible_name_programmatically

I'm also seeing hte same result when I try set the active descendant programmatically - this causes `getAttribute()` to return an empty string.

Is "sprouting" of attributes from properties supported?

According to [the explainer](https://wicg.github.io/aom/aria-reflection-explainer.html#multi-valued-array-relationship-attributes-are-supported), we should be able to set attributes programmatically for `element.ariaLabelledByElements` and then get back the references using `getAttribute()` - provided the referenced elements are in the DOM and have an id.

This does not appear to work, at least on MDN. This code does not log the references:

```html
<span id="label_1">Street name</span>
<input />
<span id="label_2">(just the name, no "Street" or "Road" or "Place")</span>
```

```js
const inputElement = document.querySelector("input");
const label1Element = document.querySelector("#label_1");
const label2Element = document.querySelector("#label_2");

// Feature test for ariaLabelledByElements
if ("ariaLabelledByElements" in Element.prototype) {
  inputElement.ariaLabelledByElements = [label1Element, label2Element];

  log(`Labeled elements: ${inputElement.getAttribute("aria-labelledby")}`);
}
```
You can see this in action as the second example in https://pr38678.review.mdn.allizom.net/en-US/docs/Web/API/Element/ariaLabelledByElements#assign_the_accessible_name_programmatically

I'm also seeing the  same result when I try set the active descendant programmatically - this causes `getAttribute()` to return an empty string.